Why are RNNs used in some computer vision problems?
There are tasks in computer vision where recurrent neural networks (RNNs) can be useful because there's some sequential sub-task in the main task.
For instance, in the paper Long-Term Recurrent Convolutional Networks for Visual Recognition and Description, the authors investigate the use of a neural network that is both recurrent and convolutional to solve certain computer vision tasks that also have a sequential component/part, such as video recognition tasks, image to sentence generation problems, and video narration challenges.
There are other papers that investigate the combination of convolutional and recurrent layers, such as Recurrent Convolutional Neural Networks: A Better Model of Biological Object Recognition, which also has a biological motivation.